1. Field of the Invention
The present invention relates to an optical switch used in optical communication and image forming apparatus for cameras, displays and the like, an encoder for distance measurement of nanometric accuracy, and a light modulating apparatus, a movement detecting device and an alignment device used in gap control, alignment and the like between a mask and a wafer in a semiconductor aligner, and the like.
2. Related Background Art
The integration degree of semiconductor devices such as LSI goes on high density, and an aligner using an excimer laser or an X-ray stepper have been utilized at present. As aligners of the next generation for foaming minuter patterns, apparatus using an F2 laser beam, electron beam, EUV radiation or near-field radiation have been investigated. In such aligners, line width resolution of 0.1 xcexcm or shorter is going to be made. However, the relative alignment accuracy between a mask and a wafer required therefor is required to be one tenth thereof, i.e. 10 nm or less at the minimum. As an alignment method of such high accuracy, a double diffraction grating method has heretofore been proposed [Flanders et al., Appl. Phys. Lett., Vol. 31, p. 426 (1977)].
The principle of this method is shown in FIG. 20. In this method, a diffraction grating is provided on each of a mask 2001 and a wafer 2002, and a laser beam 2003 is incident on these diffraction gratings to detect the diffracted ray intensity of a plurality of diffracted rays from each of the diffraction gratings, thereby detecting relative misregistration between the mask and the wafer to conduct relative alignment. Incidentally, in FIG. 20, reference numerals and characters 2004, 2005, 2006, 2007, 2008, 2009, P, d and Z indicate a grating, a mark, a detector 1, a detector 2, a device for totalizing I, an alignment signal, a pitch, a displacement and a gap, respectively.
On the other hand, there has been proposed an apparatus in which light transmission is enhanced in the case where an array of apertures provided on a metal thin film are arranged at a selected period as to the wavelength of incident light (Japanese Patent Application Laid-Open No. 11-072607). In this proposal, it is described that when the size of each aperture is from 150 nm to 1 xcexcm, and a pitch between the apertures is from 0.6 to 1.8 xcexcm, in an array of aperture of a specific pitch corresponding to the wavelength (0.5 to 1.0 xcexcm) of light incident on a portion at which the apertures are arranged, light more than the quantity of light incident on an aperture portion is transmitted through the array of apertures. It is described herein that the intensity of the light transmitted has a peak according to the correlation between the period of the array of apertures and the wavelength and becomes strong when xcex/P (xcex: the wavelength; P: the period of the array of apertures) is under certain conditions, and moreover the peak appears periodically when the wavelength of the incident light is continuously changed.
The double diffraction grating method is sensitive to a wavelength variation of a laser used in alignment, and a signal component by the wavelength variation of the laser overlaps a detected signal of the quantity of lateral misregistration. In addition, since it is necessary to use coherent light, a speckle noise by stray light overlaps, and so resolving power for the detection of the quantity of misregistration is lowered.
The phenomenon described in Japanese Patent Application Laid-Open No. 11-072607 is a phenomenon heretofore known as Wood""s anomaly caused by the fact that a grating space of a diffraction grating gradually comes near to the wavelength, whereby reflected light deviates from an originally thinkable diffraction efficiency and concentrates on zero-order diffracted light [reference literature: xe2x80x9cHikari no Enpitsu (Optical Pencil)xe2x80x9d, Masao Tsuruta (1984); and xe2x80x9cElectromagnetic Theory of Gratingsxe2x80x9d, R. Petit (1980)]. More specifically, the phenomenon is a phenomenon caused at the time the relationship between the aperture period and the wavelength satisfies certain periodic conditions since the intensity of the light transmitted through the array of apertures changes periodically according to the wavelength of the light, and in a resonance region between the array of apertures and the incident light and has involved the following problems.
First, since it is the phenomenon in the resonance region described above, the size of each aperture is limited to about {fraction (1/2)} to 2 times as large as the wavelength of light used, and the resolving power is also limited to this extent.
Second, since such resonance conditions must be satisfied, the production accuracy of the aperture size against the light wavelength becomes severe, and the production cost of the apertures becomes expensive.
Third, in this phenomenon, increase in transmitted light is a phenomenon in a far field observed by propagation light, and this method cannot be applied to a near-field region.
It is an object of the present invention to solve the above problems and provide a light modulating apparatus which can be applied to a near-field region, realize light modulation free of dependence on wavelength and permits realizing high-accuracy light modulation free of any noise cause by wavelength variations even when incoherent light little in noise cause or a laser beam is used, an optical switch using the apparatus, a movement detecting device, a distance measuring device and an alignment device using the detecting device, a semiconductor aligner using the alignment device, and processes thereof.
An example of the present invention for achieving the above object is as follows.
According to the present invention, there is provided a light modulating apparatus comprising:
first and second two periodic structures each having a period smaller than the wavelength of light emitted from a light source; and
a moving means for relatively moving the two periodic structures,
wherein the surface of the first periodic structure is brought near to the surface of the second periodic structure to a space not longer than the wavelength to arrange them in a state opposed to each other, the light incident on the first periodic structure is converted into near-field light by the first periodic structure, the converted near-field light is transmitted through the second periodic structure and converted into propagation light by scattering the near-field light on the back surface of the second periodic structure, and the intensity of the propagation light is modulated by relatively moving the two periodic structures by the moving means.
The object is achieved by a light modulating process, an optical switch, movement detecting process and device, distance measuring process and device, aligning process and device, and a semiconductor aligner using the above-described principle.
The details thereof will be described by the following embodiments and EXAMPLES.